1. Field of the Invention
This invention relates to a method and a system for handing over a connection of a mobile entity between two network access entities, in case a global address of one or both of the participating network access entities is not known to the mobile entity performing the handover.
2. Description of the Related Art
This invention is related to mobile IP networks, and in particular to performing a handover or a movement from one Access Router to another Access Router.
The invention relates to optimized IP-layer handovers (i.e. optimizations to Mobile IPv6) for seamless session mobility. More specifically, the invention is applicable for seamless session continuity during Inter-System handovers or complementary access IP layer handovers (e.g. seamless session continuity between WLAN and 3GPP systems as described in 3GPP TR 22.934 “Feasibility study on 3GPP system to Wireless Local Area Network (WLAN) interworking” (TR 22.934 v 5.1.0, December 2002)).
The IETF (Internet Engineering Task Force) is putting significant effort in the standardization of mobile solutions for IP (Internet Protocol) based networks, such as Mobile IP. The solution introduced by these standards may be complemented with other mechanisms, which are being also developed by IETF, in order to enhance the handover performance. For example, the Fast Handover Internet Draft “draft-ietf-mobileip-fast-mipv6-06.txt” (published on Mar. 1, 2003 (Work in progress)), may be used together with Mobile IPv6 to enhance the performance of the IP handover.
Further detailed information concerning Fast Handover can be found in the following documents, for example: “FAST HANDOVERS FOR MOBILE IPv6”, EURESCOM Participants in Project P1113 by Sebastien Auvray, France Telecom; “Fast Handovers and Context Transfers in Mobile Networks” by Rajeev Koodli and Charles E. Perkins (Computer Communication Review, volume 31, number 5, October 2001. ISSN #0146-4833).; and “An Analysis of The Fast Handovers for Mobile IPv6 Protocol” by Janne Lundberg, Helsinki University of Technology, Laboratory for Theoretical Computer Science, May 28, 2003.
These solutions are designed independently of the underlying technology placed below the IP layer. Therefore they could be used for implementing an IP handover between two access technologies as far as both networks, previous access network and target access network, utilize the IP protocol at the network layer. A typical example is mobility across WLAN (Wireless Local Area Network) and GPRS (General Packet Radio Service) networks. Some access technologies (such as GPRS), however, exhibit certain characteristics that may have an impact on the functionality of the IP handover as will be explained in the following discussion.
During a normal session, a Mobile Node (MN) is attached to an Access Router (AR). An Access Router offers IP connectivity to mobile nodes and acts as a default router to the mobile nodes it is currently serving. A serving Access router is also referred to as a SAR (Serving Access Router). The Access Router may include intelligence beyond a simple forwarding service offered by ordinary IP routers. In case the MN wishes to perform a handover, there are usually some Access Routers to which the MN may perform a handover. These Access Routers are referred to as Candidate Access Router (CAR).
The Access Router which is chosen for the handover is referred to as Target Access Router (TAR). In detail, the TAR is the AR with which the procedures for the MN's IP level handover are actually initiated. The TAR is selected after running a TAR selection algorithm that may take into account parameters such as the capabilities of CARs, preference of the MN and any local policies. After the handover, the TAR becomes the (new) SAR.
After performing the handover, the old SAR, to which the MN was attached before, is referred to as the Previous Access Router (PAR), which is occasionally also referred to as the Old Access Router (OAR). The PAR is the (old) SAR that will cease or has ceased to offer connectivity to the MN.
Typically a MN is connected to the AR via an Access Point (AP). An Access Point is a layer 2 device which is connected to one or more Access Routers. Access Points are sometimes called base stations or access point transceivers. An AP may be a separate from AR or co-located with an AR.
In the Fast Handover procedure, a mobile node (MN) sends an F-BU (Fast Binding Update) message when it is about to move to the TAR. Once the PAR has received the F-BU message it starts to forward the incoming packets addressed to the MN towards the TAR. The F-BU is the last message sent by the MN before leaving the PAR. Also the MN can send the F-BU message after moving to the TAR (the first message sent after the movement) if it was not possible to send it before the movement took place.
The MN always knows the link local IP address of the PAR since the MN can learn this information from the router advertisement messages received from the PAR before the movement took place. This information allows the MN to send the F-BU message to the PAR addressed to the PAR's link local IP address while the MN is still attached to the PAR's link. However it cannot be assumed that the MN always knows the PAR's publicly routable global unicast IP address. Therefore, in the cases where the MN does not know the PAR's globally routable IP address, the MN will not be able to send the F-BU message to the PAR after it moves to the target network since it will not be able to address the F-BU properly (i.e. to the PAR's globally routable IP address).
This scenario is always present when the MN has to send the F-BU after moving to the target network and the previous access network is GPRS since the globally routable IP address of the GGSN (Gateway GPRS Support Node) is totally unknown for the GPRS UE's (it is assumed that GGSN acts as Access Router for the GPRS network).
The Fast Handover Internet Draft assumes that the MN knows the PAR's globally routable IP address. This assumption does not always apply, for example, when the previous access technology is GPRS (and as a result, the PAR is GGSN). No prior solution is known so far.
The above-mentioned problem, i.e., that the globally routable addresses of Access Routers participating in a handover are not known, has also an effect on other details of the handover procedure.
Several protocols are being designed (in IETF) for seamless IP-level handovers, such as Fast handovers and Context Transfer. Since these protocols constitute IP signaling between the current Access Router (AR) and Target Access Router (TAR), a critical requirement for these mechanisms to work is that the TAR for the Mobile Node's (MN) handover is known to the current AR (FIG. 1). The TAR identification problem is being studied in the IETF Seamoby WG (Workgroup) and is subdivided as follows:                Identification of the neighboring ARs in advance of handover (HO). This procedure is also known as Candidate Access Router discovery (CAR).        TAR selection (from the list of CARs) at time of HO.        
These mechanisms require that the TAR IP address is known for current AR in order to perform a handover. In some situations, this may not be possible/desired for any of the following reasons:                The TAR resides in a different administrative domain which wants to keep its internal addressing information confidential from other administrative domains.        The TAR resides in a private IP addressing domain (i.e. the TAR does not have a publicly routable IP address).        Determination of the TAR requires some access-technology specific procedures.        
Hence, in these cases it is not possible to perform a handover to the TAR.